The present invention refers to a rotary feeder for granulated bulk material.
In general, a rotary feeder includes a rotor set in a cylindrical bore of a housing with a feed inlet which ends in a feed inlet cross section and a discharge outlet which communicates with a processing user of the bulk material. The rotor is fixed on a rotor shaft and supports a plurality of radial blades to form a plurality of compartments by which the bulk material is transported from the feed inlet top the discharge outlet.
U.S. Pat. No. 4,823,993 describes a rotary feeder for granulated bulk material, with the feed inlet of the housing including a roof-shaped baffle plate which extends at a right angle to the rotor shaft and subdivides the feed inlet cross section in two equal cross sectional areas so that a sloped cavity is formed in the rotor compartments beneath the roof-shaped baffle plate. In running direction of the rotor, the roof-shaped baffle plate ends at an inclined surface of the feed inlet which extends about two grain diameters above the cylindrical generatrix defined by the rotating, radially outer end faces of the rotor blades and ends in a pre-stripping edge which in accordance with one embodiment as shown in FIG. 8 includes two symmetrical sections. In plan view, these symmetrical sections resemble the V-shaped legs of an arrow pointing in running direction of the rotor. Extending parallel to the pre-stripping edge sections are respective stripping edges which lie in the cylindrical generatrix of the rotor and also resemble the configuration of an arrow in running direction of the rotor.
The pre-stripping edge sections and the stripping edge sections define with the cylindrical generatrix of the rotor an angle which is at least equal to the angle of friction between the bulk material and the material from which the rotor housing is made. In this manner, the fraction of granulate particles which after passing the pre-stripping edge is still retained in the pocket-like recess between the pre-stripping edge and the stripping edge, i.e. outside the generatrix of the rotor, is stripped into the sloped cavity created by the roof-shaped baffle plate within this generatrix. Thus, a shearing of grains of bulk material between the leading edge of the end faces of the rotor blades and the first housing edge being passed by the rotor blades is essentially avoided in the range of the nominal speed of the rotor and a jam-free operation of the rotary feeder is accomplished.
When viewing the sloped cavity at any given location of the rotor compartment, this cavity becomes, however, steadily smaller with increasing approximation to the V-shaped tip of the stripping edge because the cavity becomes increasingly filled with stripped granulate material and the slope conditions become increasingly unfavorable since the V-shaped tip of the stripping edge lies in rotational direction of the rotor noticeably lower than the highest surface line of the housing bore. Practice has also shown that in particular with decreasing speed of the rotor, an essentially jam-free operation could not be ensured any more. Thus, the rotary feeder of this type is only suitable to a limited degree for a dispensing operation requiring a wide range of different speeds.
German Patent No. 4,004,415 discloses a rotary feeder for granulated bulk material which differs from the previously described rotary feeder, at otherwise similar construction of the housing, in the omission of a roof-shaped baffle plate in the feed inlet i.e. at the level of the feed inlet cross section and in that the pre-stripping edge is not of arrowed configuration but extends parallel to the rotor blades. Disposed in the pocket-like recess between the pre-stripping edge and the split stripping edge is a baffle plate or deflector which is essentially of V-shaped configuration, with the tip pointing in opposition to the rotational direction of the rotor. The deflector ends at a distance to the stripping edge sections so as to define a gap of a width which is a multiple of the mean grain diameter.
A rotary feeder of this type ensures in the range of the nominal speed of the rotor a relative jam-free operation only when the filling degree of the compartments remains clearly under 100% in order to provide a sufficient volume for receiving stripped grains. Such conditions are met e.g. at operation with high speed, at pre-dispensed bulk material supply and/or at high counterpressure and a resulting leakage air flow in opposition to the introduced bulk material.